The present invention relates to head-up displays for automobiles, and more particularly to a display system for displaying substantially ghost-free images.
Head-up holographic instrument displays for vehicles have been used to provide virtual images that appear to be located ahead of the vehicle windshield toward the front of the vehicle. These displays provide the advantage of increased safety since the operator does not have to significantly divert attention from viewing the outside to check the instrument status, and are more readily noticed in the event of the display warning of a malfunction.
An example of a dynamic head-up display, i.e., one wherein the visible image is changeable, is the head-up display recently produced on certain automobiles marketed by General Motors Corporation. This display includes a head-up display unit located on top of the dashboard on the driver's side. The head-up display unit includes an image source, comprising a vacuum fluorescent display, and a curved mirror which projects the image light onto a conventional windshield. The image light reflects off the windshield and toward the driver, who sees a virtual image floating in space beyond the windshield. The curvature of the mirror causes the image to be magnified and projected about six feet beyond the windshield as a virtual image.
Most conventional windshields are made by placing a layer of polyvinylbutyral (PVB) which has a uniform thickness between two windshield singlets, and laminating the sandwiched assembly in a windshield autoclave.
In a conventional automotive head-up display (HUD) where the image source is located away from the windshield, the light from the image source reflects off the windshield toward the viewer, who sees the image floating in space beyond the windshield. If a conventional windshield is used, the viewer sees two separated images, one from the front surface of the windshield and one from the back surface. These "ghost" images interfere greatly with acceptable viewing; furthermore, the individual images themselves may not be bright enough against the ambient background.
Current approaches to reduce the ghost image problem have primarily involved the addition of a zero-degree hologram or dielectric coating on one of the inside windshield singlet surfaces adjacent the layer of PVB. These applications in effect create a third ghost image, but hopefully sufficiently reduce the brightness of the image off the outside surface of the windshield laminate so that when the brightness of the image source is adjusted properly, the ghost image from the outside surface will blend into the background, leaving the other two images. Since the thickness of a singlet is only 70-90 mils compared to the total windshield thickness of 170-210 mils, the ghost image separation of the remaining two images is smaller, and hopefully small enough to yield a substantially overlapped, acceptably viewable image.
Other approaches to reduce ghost images place a p-polarizer in front of the image source, so that the reflection off the hologram or coating is substantially larger than off the glass/air surfaces.
None of these approaches are ideal, and each of them have at least several of the following drawbacks: see-through distortion, see-through discoloration, reflection distortion, low see-through transmission, deviation from the federal transmittance specification for vehicle windshields, relatively high transmission of infrared light, poor PVB adhesion, too much PVB adhesion, and inadequate reduction of the ghost image problem. In addition, these approaches involve the addition of at least one extra component into the windshield, and therefore add to the windshield cost and complexity.